The invention relates to a method of forecasting acoustic and vibrational levels which are obtained inside a vehicle travelling over smooth ground, this vehicle being equipped with mounted assemblies composed of tires mounted on their mounting wheels.
Despite all care taken in the manufacture of the different elements making up an assembly (tire and wheel) and in the assembly thereof before they are mounted on a vehicle, it is known that any mounted assembly has a greater or lesser number of irregularities or non-uniformities when the wheel turns. When the vehicle is travelling, these irregularities or non-uniformities cause vibrational excitation of the different components of a vehicle when the said vehicle is equipped with the said assembly, in ways which may be different and vary in dependence on the speed of the said vehicle.
By non-uniformity of a mounted assembly or a rotating assembly there is meant an irregularity which may be connected among other things with:
one or more faults or irregularities in the distribution of masses or rigidity in the mounted assembly;
eccentric mounting of the wheel on the vehicle;
geometric faults in the wheel;
poor mounting of the tire on its wheel.
These irregularities lead to cyclic variations in the forces on the wheel centers and/or at any other point of attachment of the system connecting the vehicle to the ground. By a system connecting to the ground there is meant a sub-assembly of elements of the vehicle ensuring that the vehicle body is connected to one or more surfaces of contact between the vehicle and the ground. This sub-assembly is determined in advance and comprises for example all the mechanical parts of the vehicle located between at least one fixing point on the body of the said vehicle and at least one footprint of each tire with which the said vehicle is equipped; in this example, at least one tire is included in the system connecting to the ground.
These cyclic variations of forces resulting from the non-uniformities become evident during travel in vibrations of the floor of the vehicle, the seats and the steering wheel. An acoustic disturbance may also occur with noise being created inside the vehicle by the vibrations of different parts of the said vehicle. The level of nuisance perceived depends on the vehicle and the speed of travel (there are also critical speeds, at which the nuisance perceived is at a maximum), but mainly on the level of non-uniformity of the rotating assemblies.
In the text below, xe2x80x9ccomfort performancexe2x80x9d means the acoustic and vibrational level measured inside a given vehicle equipped with a set of mounted assemblies.
Those skilled in the art are familiar with a method of evaluating the comfort performance of a new tire for a given vehicle, in accordance with which noise and vibration measurements are made in the passenger compartment of a vehicle travelling over a section of smooth road, by working through different levels of non-uniformity (for example 10 grams, 20 grams, 30 grams of unbalanced load) for the four mounted assemblies of the vehicle over the entire range of speeds of the vehicle in order to be sure of having evaluated the maximum disturbance. This method requires the vehicle to be available, however, and can only be performed in favorable weather conditions, which gives rise to what is sometimes an excessively long immobilization period and consequently an additional cost. This method is moreover long and laborious for the operator, is subject to large variations in measurement, and is expensive in the implementation phase of a tire for a given vehicle.
The invention relates to a method of forecasting the comfort performance (level of noise and vibration) which does not have the disadvantages of the method which have just been mentioned.
With this object, there is proposed a method of forecasting the noise and vibrations in the passenger compartment of a vehicle equipped with test mounted assemblies and travelling at a given speed V over smooth ground, this method consisting in multiplying an overall transfer function of the vehicle equipped with reference mounted assemblies by forces resulting at the wheel center of the test mounted assembly, this mounted assembly bearing an identical load to that measured on the vehicle and travelling over the same ground. The method proposed includes a first step of determining an overall transfer function of the vehicle by using a reference mounted assembly and a second step in which forces are measured at the wheel center of the test mounted assembly.
By a reference mounted assembly there is meant a mounted assembly used to assess the overall transfer function, this reference mounted assembly being prepared such that it has a number N of irregularities of appropriate dimensions to excite the vehicle in a range of frequencies Fmin, Fmax within the spectrum of speeds selected (Vmin, Vmax). By a test mounted assembly there is meant a mounted assembly whereof the comfort performance is to be forecast for the vehicle, taking into account the non-uniformities this test mounted assembly may have. These mounted assemblies (test and reference) are composed of a wheel provided to be mounted on the vehicle (in accordance with the constructor""s specifications) and a tire, also provided to be mounted on the vehicle. As a general rule, the mounting wheel has parts forming seats and parts forming edges, these parts being intended to cooperate with beads of the tire when the latter is mounted on this wheel. Moreover, the tire comprises a tread whereof the radially outer surface is intended to come into contact with the roadway during travel; sidewalls connect the tread to the beads.
Principle of the Method.
There is proposed a method of forecasting the comfort performance (noise and vibrations) in a vehicle equipped with a test mounted assembly in a selected position on a vehicle travelling at a given speed V on a travel medium of the smooth ground type, allowing an equivalent maximum speed Vmax of the vehicle to be attained (Vmax being greater than the given speed V), this test mounted assembly having a greater or smaller number of faults of non-uniformity as mentioned at the beginning of this description.
This method comprises the following steps:
(a)xe2x80x94preparing a reference mounted assembly comprising a tire and its mounting wheel, such that this reference mounted assembly of circumferential length xcfx80D has a number N of irregularities which are substantially identical and regularly distributed over the circumference of the mounted assembly, these irregularities being selected to excite the vehicle in a range of frequencies from a minimum frequency Fmin to a maximum frequency Fmax in the course of travel over smooth ground, this number N being determined by the formula:   N  =            integer      ⁢              xe2x80x83            ⁢      part      ⁢              xe2x80x83            ⁢              (                              (                          F              ⁢                              xe2x80x83                            ⁢              max              xc3x97              π              ⁢                              xe2x80x83                            ⁢              D                        )                                V            ⁢                          xe2x80x83                        ⁢            max                          )              +    1  
where: Fmax is in hertz, D is in meters and Vmax is in meters per second;
(b)xe2x80x94equipping the vehicle with the reference mounted assembly in the selected position on the vehicle;
(c)xe2x80x94positioning the vehicle such that the reference mounted assembly rests on the travel medium capable of causing the said reference mounted assembly to rotate;
(d)xe2x80x94recording the noise and vibrations at at least one point inside the vehicle by working through the speeds of rotation of the reference mounted assembly in a range of speeds of rotation (xcfx89min, xcfx89max) equivalent to the speeds (Vmin, Vmax) of the vehicle, the speed Vmin being defined by the following equation:       V    ⁢          xe2x80x83        ⁢    min    =            (              F        ⁢                  xe2x80x83                ⁢        min        xc3x97        π        ⁢                  xe2x80x83                ⁢        D            )        N  
(e)xe2x80x94measuring the resulting forces exerted at the wheel center of the reference mounted assembly, this assembly being mounted to travel either on the travel medium used in step (c) or on a travel medium equivalent to that used in step (c) and put under the conditions of travel (direction of rotation, tire pressure, load, range of speeds of rotation) corresponding to the position selected on the vehicle;
(f)xe2x80x94calculating for this position on the vehicle an overall transfer function between the forces measured at the wheel center of the reference mounted assembly and the noise and vibrations in the vehicle;
(g)xe2x80x94measuring the resulting forces at the wheel center of the test mounted assembly travelling either on the travel medium from step (c) or on a travel medium equivalent to that used in step (c), under conditions of tire pressure and load identical to those used in step (c) at a speed of rotation equivalent to the speed V;
(h)xe2x80x94multiplying the forces obtained in step (g) by the overall transfer function calculated in step (f) to obtain a forecast of the noise and vibrations in the vehicle travelling at speed V, the said vehicle being equipped with the test mounted assembly in the selected position.
By travel medium there is meant an assembly of media allowing a mounted assembly to be put under the appropriate loading condition and the said mounted assembly to be driven in rotation, in particular a testing drum, a roller bench or indeed a testing machine having a flat track passing below the tire of the mounted assembly.
In order to obtain a spectrum extending over the entire range of frequencies selected, the measurements are performed by working through the speeds of rotation of the reference mounted assembly. This working through the speeds may be carried out continuously by accelerating between the minimum speed and the maximum speed or indeed by decelerating between these two speeds; this working through may also be carried out for a plurality of speeds between these minimum and maximum speeds.
One advantage of the present method lies in the fact that the forces involved in determining the overall transfer function and in using it later to characterize a new mounted assembly are effectively the actual forces at the wheel center.
The measurements described in steps (e) and (g) above are provided to be performed at the wheel center of the reference mounted assembly and test mounted assembly; it goes without saying that it is possible to perform them while taking into account not just one mounted assembly but a more complex system including both the said mounted assembly and particular elements connecting the said assembly to the vehicle, as for example suspension elements. In this case, the points where the measurements are made correspond to the points of attachment of these particular elements to the vehicle.
The method fulfils the object, which is to evaluate for a given vehicle the comfort performance of a mounted assembly having a given level of non-uniformity. With the proposed method, it is not necessary to have the vehicle available for each new mounted assembly differing from the reference mounted assemblies, since the overall transfer function of the vehicle is established on a travel medium which is also the travel medium used to measure the forces at the wheel center of the mounted assembly by itself (or at any other point of attachment selected for measurement).
Moreover, and surprisingly, it has been found that it was possible for a given vehicle to go beyond the dimension of the tire. This means that for a vehicle provided to be equipped with different tire dimensions (for example between the front axle and the rear axle), it is possible to determine an overall transfer function which is applicable to all the dimensions accepted by the said vehicle.
By overall transfer function there is meant a function comprising two parts, a first part relating to the noise in the passenger compartment of the vehicle and a second part relating to the vibrations at certain predetermined points in the same passenger compartment. The determination of the overall transfer function is given by using a series of measurements on the vehicle equipped with the reference mounted assembly and a series of measurements on the reference mounted assembly by itself, these measurements corresponding to the measurements as presented in the principle of the method.
In practice, with a single tire and a single wheel, it will be possible to create a plurality of different test mounted assemblies by modifying the mounting of the said tire on its wheel (that is to say the positioning of the tire on its wheel), or the balancing of the assembly (that is to say the distribution of the rotating masses), or indeed the mounting of the assembly on the axle. By means of these modifications, the non-uniformity of the mounted assembly is changed, which can of course be evaluated by recording the forces at the wheel center; use of the method according to the invention results in a forecast of the noise and vibrations in the vehicle resulting from this change.
In order to operate between a zero frequency and the minimum frequency Fmin, an unbalanced load is provided on the reference mounted assembly. After balancing this reference mounted assembly, a particular mass of between 30 and 50 grams is fixed next to the valve, in order to create a dynamic unbalanced load (that is to say a disequilibrium in the distribution of the rotating masses).
First Embodiment of the Irregularities in the Reference Mounted Assembly.
On a tire provided with an initially smooth tread there are made a plurality of substantially identical irregularities distributed regularly over the circumference of the tire tread. These irregularities comprise a series of notches having substantially the same width and a depth between 0.5 and 3 mm; these notches extend over the entire width of the tread. Preferably, the notches form an angle at most equal to 30xc2x0 with the axis of rotation of the tire.
These indentations may be made by cutting the molded vulcanized tire or by molding in the course of molding the tire.
The number of notches N, defined by the formula below, is a function of the maximum frequency selected and the maximum permitted speed for the vehicle:   N  =            integer      ⁢              xe2x80x83            ⁢      part      ⁢              xe2x80x83            ⁢              (                              (                          F              ⁢                              xe2x80x83                            ⁢              max              xc3x97              π              ⁢                              xe2x80x83                            ⁢              D                        )                                V            ⁢                          xe2x80x83                        ⁢            max                          )              +    1  
where: Fmax is in Hz
D is in meters
Vmax is in meters/second.
Preferably, the notches of mean width Le are disposed regularly in the circumferential direction at a mean pitch P, the relationship between the mean width Le and the pitch P being determined by the following equation:
0.05xe2x89xa6Kxe2x89xa65
      where    ⁢          xe2x80x83        ⁢    K    =      Le          P      -      Le      
When this ratio K is less than 0.05, the level of non-uniformity is not sufficient, but when this same ratio is greater than 5, the rigidity of the tread is excessively affected thereby.
Second Embodiment of the Irregularities of the Reference Mounted Assembly.
Another variant consists in positioning on the tread (whether the latter is smooth or sculptured) a series of extra-thick portions extending in the transverse direction (at most 30xc2x0 with respect to the axial direction of the tire).
Third Embodiment of the Irregularities.
A plurality of N insert blocks of the same width are placed uniformly distributed between an edge of the rim and the tire bead. These insert blocks are from 4 to 6 mm thick and extend in the radial direction over the entire height of the edge. It goes without saying that it is possible to increase the effect of non-uniformity by providing insert blocks on both edges.
Fourth Embodiment of the Irregularities.
A plurality of N insert blocks of the same thickness and the same width are arranged uniformly between at least one of the tire beads and the seat of its mounting wheel.
Fifth Embodiment of the Irregularities.
The rigidity of the carcass reinforcement in the sidewalls of the tire is modified, for example by making N sectors having cuts and strengthened portions in this reinforcement, these N sectors being uniformly distributed in the circumferential direction, while taking care to maintain the leak-tightness of the said tire.